Polymeric additives such as viscosity index improvers are currently used in many applications such as multigrade engine oils and many transmission and hydraulic oils. These polymeric additives may function to modify the bulk rheological properties of the lubricant fluids. It is believed that some of these additives adsorb onto lubricated metallic surfaces, form films, and reduce friction between two contacting parts in moving relation to one another. The adsorbed layers may maintain a hydrodynamic film, and thereby correspondingly lower friction. Such a film may be present even at low entrainment speeds at which hydrodynamic films may not be expected based on the viscosity of the bulk lubricant.
Fan et. al (J. Fan, M. Muller, T. Stohr, H. A. Spikes; Tribology Letters (2007) 28:287-298) have shown polymeric additives containing functionalized methacrylates (f-PAMAs) which have functional groups that may be able to adsorb on polar solid surfaces such as engineering steel. In their work, they used functional groups that may be clustered within the polymer molecule, for example in a block distribution, rather than in a random distributed along the polymer chain. Some of these polymers may be characterized as diblock copolymers containing the functionalized polymethacrylate block and a nonpolar block (e.g., a nonpolar tail) which may protrude from the surface being lubricated into the lubricating fluid. The film thicknesses ranged from 10 and 30 nm, increasing with the molecular weight.
Generally, the control of film thickness by increasing the polymer molecular weight may be limited by solubility as shown by S. M. Hsu*, R. S. Gates (Tribology International 38, 2005, 305-312). Poor shear stability of linear molecules, particularly under high temperature and high shear rate conditions have also been observed (see e.g., I-Chun Liou, Raymond Chien-Chao Tsiang, James Wu, Jin-Shang Liou, Hun-Chang Sheu Journal of Applied Polymer Science, Vol. 83, 2002, 1911-1918). The limit to film thickness with polymer additive adsorption and the potential inferior wear protection when the surface roughness exceeds a certain level is discussed by (R. P. Glovnea, A. V. Olver, H. A. Spikes, “Lubrication of Rough Surfaces by a Boundary Film-Forming Viscosity Modifier Additive”, Journal of Tribology 2005, 127: 223-229).
The use of particles consisting of alkaline metal carbonates or hydroxides has been discussed by Bakunin et al. (V. N. Bakunin, A. Yu. Suslov, G. N. Kuzmina and O. P. Parenago A. V. Topchiev Journal of Nanoparticle Research 6: 273-284, 2004). These may be so-called overbased dispersing additives, and may improve corrosion resistance by neutralizing acids from the oxidation reactions (carboxylic acids) or sulfuric acids from fuel combustion process. Such particles containing carbonates of Alkali and Alkali earth metals are further described by Waynick (PCT Patent Application Publication Number WO 2006/119502, published Sep. 11, 2006).
Lubricating particles that have a flat planar structure, such as MoS2, graphite and WS2 have been used in lubricants. These particles may be stabilized as a suspension in the lubricant fluid as discussed by Coyle et al. (U.S. Pat. No. 4,995,996, issued Feb. 26, 1991, expressly incorporated herein by reference) using xanthates; Migdal et al. (U.S. Pat. No. 6,878,676, issued Apr. 12, 2005), using molybdenum sulfide particles which are capped with different non polymeric compounds such as alkyl amines, dialkyl amines, trialkyl amines, carboxylic acids; and Malshe et al. (PCT Patent Publication Number WO 2007/082299, published Jul. 19, 2007), using particles which are treated with surfactants such as lecitin, phospholipids, detergents, and sorbitan esters of fatty acids.
Colloidal suspensions having elemental metallic core particles of bismuth, tin, zinc, copper, and silver have been described by Kernizan et al. (U.S. Pat. No. 6,613,721, issued Sep. 2, 2003) where the surfactant stabilized particles may coat the surface requiring lubrication and fill in surface asperities.
Liu (PCT Patent Application Publication No. WO 2007/068102, published Jun. 21, 2007) uses organic particles stabilized by diblock or triblock copolymers. The polymer blocks provide limited attachment to a surface that is being lubricated as they contain metal binding groups that are scattered within the blocks.
FIGS. 1A and 1B illustrate features of a number of the prior art lubricant additives.
There continues to be a need for improved lubricants, such as lubricants that can contain additives that improve the performance of the lubricant. For example, there is a need for lubricants having a longer service life, e.g., when exposed to conditions of high shear, high temperature, or both. There is also a need for new lubricants and lubricant additives that reduce the wear of lubricated parts. Furthermore, there is a need for improved lubricants and lubricant additives which reduce friction (e.g., in intermediate and slow entrainment speed conditions), form a stable suspension of particles in the lubricant, form a stronger attachment to the surface being lubricated, or are more selective in locating the part that needs to be lubricated. Other attractive features desired for improved lubricants also include the ability to form a thicker lubricating layer. For example, it may be desired for a lubricant to contain an improved lubricant additives which result in a thicker lubricating layer.
There continues to be a need for lubricant additives which combine multiple features such as a combination of any two or more features for which a need was above described. For example, there is a need for lubricants additives which contain two, three or all of the features selected from the group consisting of antiwear additive, heat stabilizer additive, antioxidant, and viscosity reducer.